1. Field of the Invention
This invention relates to a method and an apparatus for pretreating natural gas streams entering a liquefied natural gas LNG plant. In another aspect the invention concerns a natural gas pretreatment system that uses process heat integration to control the natural gas temperature during pretreatment and lower the temperature of the natural gas entering the LNG plant.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
With regard to ease of storage, natural gas is frequently transported by pipeline from the source of supply to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to xe2x88x92240xc2x0 F. to xe2x88x92260xc2x0 F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
Prior to liquefying a natural gas stream in a LNG plant, the natural gas stream must be pretreated to remove components such as water, acid gases, heavy (C3+) hydrocarbons, and mercury. Such pretreatment is typically accomplished either upstream of all the chilling stages or immediately downstream of an initial chilling stage. It is well known that different pretreatment steps are more effective and efficient at different temperatures and pressures. For example, liquid separation is best accomplished at lower natural gas temperatures while acid gas removal is best accomplished at higher natural gas temperatures. Further, if the temperature of the natural gas stream can be sufficiently lowered during pretreatment, it may be possible to eliminate one or more initial chilling stages of the liquefaction process. Thus, a pretreatment system that more effectively and efficiently controls the temperature of the natural gas throughout pretreatment while lowering the overall temperature of the natural gas during pretreatment would be a significant contribution to the art and to the economy.
It is, therefore, an object of the present invention to provide a novel system for pretreating a natural gas stream entering a natural gas liquefaction plant that provides for more efficient control of the natural gas temperature and pressure.
Another object of the present invention is to provide a novel natural gas pretreatment system utilizing process heat integration to more efficiently and effectively control the temperature of the natural gas during pretreatment.
Still another object of the present invention is to provide a novel natural gas pretreatment system that employs an expander for lowering the pressure of the natural gas stream and for producing energy used elsewhere in the LNG plant.
A further object of the present invention is to provide a novel natural gas pretreatment system wherein the temperature of the natural gas stream exiting the pretreatment system is significantly lower than the temperature of the natural gas stream entering the pretreatment system.
A still further object of the present invention is to provide a natural gas pretreatment system that lowers the pressure of the natural gas in an initial pretreatment step, thereby allowing lower pressure rated equipment to be employed in the pretreatment system.
It should be noted that the above-listed objects and advantages of the invention are exemplary only, and other objects and advantages of the invention will be apparent from the written description and drawings.
Accordingly, in one embodiment of the present invention, there is provided a pretreatment process for natural gas liquefaction comprising the steps of: (a) heating a natural gas stream in a first side of a first heat exchanger; (b) downstream of the first side of the first heat exchanger, heating the natural gas stream in a first side of a second heat exchanger; (c) downstream of the first side of the second heat exchanger, removing an acid gas from the natural gas stream in an acid gas removal system; (d) downstream of the acid gas removal system, cooling the natural gas in a second side of the second heat exchanger by indirect heat exchange with the natural gas stream that is heated in step (b); (e) downstream of the second side of the second heat exchanger, removing water from the natural gas stream in a first dehydrator; and (f) downstream of the first dehydrator, cooling the natural gas stream in a second side of the first heat exchanger by indirect heat exchange with the natural gas that is heated in step (a).
In another embodiment of the present invention, there is provided a pretreatment process for natural gas liquefaction comprising the steps of: (a) reducing the pressure of a natural gas stream in an expander; (b) downstream of the expander, removing liquids from the natural gas in a first gas-liquid separator; and (c) downstream of the first gas-liquid separator, cooling the natural gas stream in a first chiller employing a refrigerant comprising in major portion a hydrocarbon selected from the group consisting of propane, propylene, ethane, ethylene, and combinations thereof.
In still another embodiment of the present invention, there is provided a pretreatment process comprising the steps of: (a) removing water from a natural gas stream in a first dehydrator; (b) downstream of the first dehydrator, reducing the pressure of the natural gas stream in an expander; (c) downstream of the expander, removing liquids from the natural gas stream in a first gas-liquid separator; (d) downstream of the first gas-liquid separator, heating the natural gas stream in a first side of a first heat exchanger; (e) downstream of the first side of the first heat exchanger, heating the natural gas stream in a first side of a second heat exchanger; (f) downstream of the first side of the second heat exchanger, removing an acid gas from the natural gas in an acid gas removal system; (g) downstream the acid gas removal system, cooling the natural gas in a second side of the second heat exchanger; (h) downstream of the second side of the second heat exchanger, removing liquids from the natural gas stream in a second gas-liquid separator; (i) downstream of the second gas-liquid separator, removing water from the natural gas stream in a second dehydrator; and (j) downstream of the second dehydrator, cooling the natural gas stream in a second side of the first heat exchanger.
In a further embodiment of the present invention, there is provided a pretreatment apparatus for natural gas liquefaction. The apparatus comprises an expander, a first heat exchanger, and an acid gas removal system. The expander is operable to reduce the pressure of the natural gas. The first heat exchanger defines a first fluid flow path and a second fluid flow path. The first heat exchanger is operable to facilitate indirect heat exchange between natural gas flowing through the first fluid flow path and natural gas flowing through the second fluid flow path. The inlet of the first fluid flow path is fluidly coupled to the outlet of the expander. The acid gas removal system has an inlet that is fluidly coupled to the outlet of the first flow path. The acid gas removal system has an outlet that is fluidly coupled to the inlet of the second flow path.